Apparatus and method for voltage contrast analysis of a wafer using a titled pre-charging beam

ABSTRACT

A method for electrically testing a wafer that includes: receiving a wafer having a first layer that is at least partly conductive and a second layer formed over the first layer, following production of openings in the second layer; directing towards the wafer a first set of beams of charged particles that are oriented at a first set of angles in relation to the wafer, whereas each angel of the first set of angles deviates substantially from normal, so as to pre-charge an area of the second layer without substantially pre-charging the first layer; scanning the area of the wafer by a second set of beams of charged particles that are oriented at a second set of angles in relation to the wafer, and collecting charged particles scattered from the area wafer. A system for electrically testing a semiconductor wafer, the system includes: at least one charged particle beam source; at least one detector, adapted to collect charged particles scattered from the wafer; whereas the wafer comprises a first layer that is at least partly conductive and a second layer formed over the first layer, following production of openings in the second layer; whereas the system is adapted to: (i) direct towards the wafer a first set of beams of charged particles that are oriented at a first set of angles in relation to the wafer, whereas the first angle deviates substantially from normal, so as to pre-charge an area of the second layer without substantially pre-charging the first layer; (ii) scan the area of the wafer by a second set of beams of charged particles that are oriented at a second set of angles in relation to the wafer, and collect charged particles scattered from the area wafer.

FIELD OF THE INVENTION

This invention relates to apparatus and method for inspecting andtesting semiconductors wafers during circuit fabrication and, inparticular, for testing wafers in a voltage-contrast mode, especiallyusing a tilted pre-charging beam of charged particles.

BACKGROUND OF THE INVENTION

Integrated circuits are very complex devices that include multiplelayers. Each layer may include conductive material, isolating materialand/or semi-conductive materials. These various materials are arrangedin patterns, usually in accordance with the expected functionality ofthe integrated circuit. The patterns also reflect the manufacturingprocess of the integrated circuits.

Contact hole production is a common step in semiconductor devicemanufacturing. The contact holes are typically used to make electricalconnections to a semiconductor or metal layer through an overlyingnon-conducting (dielectric) layer, such as an oxide layer. In order toproduce contact holes, a layer of photoresist is first deposited on thewafer surface. The photoresist is exposed to patterned visible orultraviolet radiation, hardened, and developed in order to form a “mask”over the wafer, with mask patterns corresponding to contact holelocations. Then the wafer is transferred to an etch station wherecontact holes are formed through the dielectric layer, down to theunderlying semiconductor or metallic layer. The photoresist mask is thenremoved, and the contact holes are filled with metal. A similar maskingand etching process is used in producing trenches or vias in the wafersurface.

In order to ensure consistent device performance, variouscharacteristics of the contact openings must be carefully controlled atvarious locations across the wafer surface. (In the context of thepresent patent application and in the claims, the term “contactopenings” refers to all structures of the type described above,including contact holes, vias, and trenches.)

In some cases the contact hole does not define a proper contiguous spacethat ends at the underlaying semiconductor or metallic layer. In otherwords, the contact hole can be at least partially filled bynon-conductive material that can alter the resistance of the conductorthat is later formed within the contact hole. The non-conductivematerial can also form a barrier between the conductive material filledwithin the defective contact hole and the underlaying layer. Such adefect can cause an electronic circuit to be electrically “open” insteadof being electrically “closed”. It can also result in higher impedancethan expected from a contract that passes through a non-defectivecontact hole.

Voltage contrast techniques facilitate a determination of electricalproperties of wafers and are based upon a detection of differentcharging conditions of different elements of an inspected sample. U.S.Pat. No. 6,627,884 of McCord, et al. titled “Simultaneous flooding andinspection for charge control in an electron beam inspection machine”,and U.S. Pat. No. 6,586,736 of McCord titled “Scanning electron beammicroscope having an electrode for controlling charge build up duringscanning of a sample”, which are incorporated herein by referencedescribe voltage contrast techniques.

Voltage contrast methods usually includes a pre-charge stage that isfollowed by a scan and image stage. Some prior art method use floodingguns that pre-charge a sample by scanning the sample with a normalincident charge particle beam. These techniques are suited to handlesamples that include grounded underlaying layers where underlayingconducting layers remain discharged after the pre-charge stage while theoxide layer is charges, thus increasing the voltage contrast betweenopen contact holes and their surrounding oxide layer.

FIG. 1 illustrates a cross section of a typical prior art SOI wafer 200The lowest layer is a substrate 210. The substrate is usually made ofsilicone. An oxide layer (also referred to as BOX) 220 is manufacturedabove the substrate 210. The upper layer of the SOI wafer includes aninter-dielectric layer 240 through which contact holes 245 werefabricated. Trench insulators, such as oxide-made trench insulators 260as well as silicone epilayer islands 230-232, that are insulated fromeach other by trench isolators 260 are formed between theinter-dielectric layer 240 and the oxide layer 220.

Various wafers such as silicon over insulator (SOI) wafers and shortloop wafers have sub surface structures that are intentionally floating.Thus, the pre-charging stage can charge both the inter-dielectric layer240, any residual material within contact holes 245, and BOX layer 220.Thus, both the efficiency of voltage techniques is greatly reduced.

There is a need to provide a system and method for an effective voltagecontrast analysis.

SUMMARY OF THE INVENTION

A method for electrically testing a semiconductor wafer, the methodincludes: receiving a wafer having a first layer that is at least partlyconductive and a second layer formed over the first layer, followingproduction of openings in the second layer; directing towards the wafera first set of beams of charged particles that are oriented at a firstset of angles in relation to the wafer, whereas each angle of the firstset of angles deviates substantially from normal, so as to pre-charge anarea of the second layer without substantially pre-charging the firstlayer; scanning the area of the wafer by a second set of beams ofcharged particle beam that are oriented at a second set of angles inrelation to the wafer, and collecting charged particles scattered fromthe area wafer.

A system for electrically testing a semiconductor wafer, the systemincludes: at least one charged particle beam source; at least onedetector, adapted to collect charged particles scattered from the wafer;whereas the wafer comprises a first layer that is at least partlyconductive and a second layer formed over the first layer, followingproduction of openings in the second layer; whereas the system isadapted to: (i) direct towards the wafer a first set of beams of chargedparticles that are oriented at a first set of angles in relation to thewafer, whereas each angle of the first set of angles deviatessubstantially from normal, so as to pre-charge an area of the secondlayer without substantially pre-charging the first layer; (ii) scan thearea of the wafer by a second set of beams of charged particles that areoriented at a second set of angles in relation to the wafer, and collectcharged particles scattered from the area wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 illustrates a cross section of a typical prior art SOI wafer;

FIG. 2 illustrates scanning electron microscope (SEM) that is capable ofvoltage contrast analysis, according to an embodiment of the invention;

FIG. 3 illustrates a SEM that includes a pre-charge gun that provides atilted pre-charge beam according to an embodiment of the invention; and

FIG. 4 is a flow chart of a method 200 for performing voltage contrastanalysis according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description relates to charged particle microscopes, suchas Scanning Electron Microscopes (SEMs), such as step and repeat typeSEMs, in which wafer is scanned by repetitive steps of scanning an areaof the wafer (said area defined by the field of view of the SEM) andmechanically introducing a movement between the wafer and SEM tofacilitate the scanning of another area. Said movement may also beimplemented by electrostatic and/or magnetic fields introduced byvarious electrostatic and/or magnetic elements such as lens, deflectorsand the like. It is noted that other charged particles and even photonsmay be utilized for detecting voltage contrast. It is further noted thatthis invention may also be implemented by introducing a substantiallyconstant movement between the SEM and the wafer. The movement may belinear or even rotational, and/or any combination of both movements.

Generally, a tilt mechanism can be implemented by mechanically tiltingeither the sample carrier relative to the charged particle beam columnor the column relative to the sample's stage. A beam can be tilted abeam by using single- or double-deflection. The use of adouble-deflection technique, namely, pre-lens and in-lens deflectionstages is known in the art.

The following descriptions refer, for convenience of explanation alone,to a system and method that use a single primary charged particle beam.

According to an embodiment of the invention multiple charged particlebeams can be used during the pre-charge stage. Alternatively oradditionally, multiple primary charged particle beams can be utilizedduring the imaging stage. Systems and methods for producing multiplecharged particle beams, either from multiple sources, or from a singlesource are known in the art.

The term “set” as used in this application may include a single member.For example, the phrase “a first set of beams of charged particles thatare oriented at a first set of angles” also includes a first beam thatis oriented at a first angle”, the “a second set of beams of chargedparticle beam that are oriented at a second set of angles” also includesa second beam that is oriented at a second angle.

It is noted that the multiple pre-charge beams can be directed towardsthe wafer at different directions, as long as they are tilted enough inrelation to the wafer.

Using multiple pre-charge beams can increase the pre-charging uniformityfor wafers with topographical structure such as but not limited to dualdamascene. For dual damascene the via/contacts are recessed intotrenches which can run in both X and Y directions.

According to another embodiment of the invention the pre-charge beam isnot tilted and a angle of incidence in relation to the wafer is achievedby very strong over focusing of the pre-charge beam. The strongover-focusing provides a mixture of normal incidence and high angleelectrons, but would be easier to design and implement than a tiltedgun. It would also give a 360 degree population of high angle electronsfor more uniform charging.

FIG. 2 illustrates a double deflection tile mechanism in which theprimary charged particle beam is tilted by deflectors positioned in aplane of an objective lens and additional deflectors are located betweenthe funnel of the objective lens and the inspected object. According toother embodiments of the invention the amount of deflectors as well astheir position can vary.

It is further noted that some SEM systems, such as the Applied MaterialsNanoSEM3D, Applied Materials G2 SEMVision provide this sort of beam tiltcapability.

Various configurations of charged particle devices including SEMs,including an apparatus that allow to tilt a charged particle beam isdescribed in the following U.S. patents and U.S. patent applications ofPetrov et al., all being incorporated herein by reference: U.S. patentapplication 20040173746 titled “Method and system for use in themonitoring of samples with a charged particles beam”, U.S. patentapplication 20030218133 titled “charged particle beam column and methodfor directing a charged particle beam”; U.S. patent application20040056207 titled “deflection method and system for use in a chargedparticle beam column”, and U.S. Pat. No. 6,674,075 titled “Charged beamapparatus and method for inspecting samples”.

The invention can be applied at SEM that includes at least one electrodethat is positioned near the wafer, but this is not necessarily so. Insuch a case the electrode has at least one opening for allowing thepre-charging beam as well as the scanning and imaging beam to passtherethrough.

The tilt angle is selected such that the pre-charging beam does notsubstantially interact with the bottom of the contact hole, but ratherinteracts with the sidewall of the contact hole that is a part of theinter-dielectric layer 240.

For example, assuming that: (i) the wafer is positioned along the X-Yplane and that openings are formed along the Z-axis, (ii) the depth(along the Z-axis) of a contact hole is H, (iii) the width (diameter, atthe X-Y plane) of a contact hole is W, then the tilt angle, or morespecifically the Z-axis trajectory of said angle α, has to fulfill thefollowing geometrical relationship: α<arctangent(W/H). W/H is also knownas the aspect ratio of the contact hole. Each angle of a first set ofangles (which may include one or more angles) shall fulfill thiscriterion.

The amount of charged particles that reaches the bottom of the contacthole can be also reduced by supplying positive voltage at the vicinityof the wafer.

FIG. 2 illustrates scanning electron microscope SEM 10 that is capableof voltage contrast analysis, according to an embodiment of theinvention. The charged particle beams are illustrated during apre-charge stage.

SEM 10 includes multiple components for providing a primary electronbeam 40. The primary electron beam 40 is generated by SEM 10 anddirected towards a wafer such as wafer 200. The primary electron beam 40is used as a pre-charging beam during a pre-charge stage and can be usedto irradiate wafer 200 during a scan and image stage. Thecharacteristics (including intensity, tilt angle, landing energy and thelike) of the primary electron beam 40 can be different from stage tostage, according to the use of said primary electron beam.

SEM 10 usually includes a large number of components such as but notlimited to an electron gun, one or more electrodes and anodes, one ormore high voltage power supply and the like. For simplicity ofexplanation only tip 12 is shown.

The primary electron beam passes through an aperture (also referred toas opening) 15 formed within detector 14 and propagated towards acomplex that includes an objective lens 16 and beam shift deflectors 18.The objective lens 16 usually includes multiple magnetic andelectro-static components such as pole-pieces, caps, coils and the like,all being known in the art but not illustrated in FIG. 2.

The path of the primary electron beam is altered twice. The beam shiftdeflectors 18 introduce a first shift while a second shift is introducedby tilt deflectors 20 that are positioned between the objective lens 16and the inspected object. Two dashed horizontal lines illustrate thecenter of each pair of deflectors and the location of the shift.Deflectors 20 and 18 deflect the primary electron beam such that it isdirected towards the object in a tilted manner. Thus, a tiltedpre-charge beam is obtained.

It is noted that during an inspection phase the primary electron beamcan be normal to the inspected object or oriented in respect to animaginary line that is normal to the inspected object.

During a scan and image stage the primary electron beam interacts withwafer 20 and as a result various types of electrons, such as secondaryelectrons, back-scattered electrons, Auger electrons and X-ray quantaare reflected or scattered. Secondary electrons can be collected easilyand most SEMs mainly detect these secondary electrons. At least some ofthe scattered electrons are subjected to the magnetic and/orelectrostatic fields introduced by objective lens 16, and deflectors 18and 20 are directed towards detector 14.

SEM 10 detects secondary electrons by detector 14. The detector 14 isconnected to controller 60 that is capable of generating an image of thescanned wafer in response to the amplitude of collected secondaryelectrons and the location of the primary electron beam in relation tothe wafer.

There are various detector configurations that can be applied. FIG. 2illustrates a single in-lens detector but SEM 10 may include multipledetectors, al least one out-of-lens detector and the like. Typically,the detector includes a detecting segment that includes an opening, butother configurations can be implemented. For example, the opening can beformed between more than a single detector segment.

SEM 10 may include an additional electrode 30 located near the inspectedobject, for improving the control of electromagnetic fields at thevicinity of the inspected object. Although FIG. 2 illustrates a singleelectrode, this is not necessarily to and multiple electrodes, as wellas an electrode that is segmented to multiple portions, can be appliedto control the charging of wafer 200.

Controller 60 is also connected to the stage 50 for controlling amechanical movement introduced between wafer 100 and other parts of theSEM 10.

Controller 60 is capable of controlling other aspects of SEM 10operation, such beam deflection, beam focusing, beam generation, and thelike. It is noted that the controller 60 can include multiple softwareand hardware components, can be a single device or multiple devices.

Conveniently, during a scan and image stage, the stage 50 moves thewafer along a Y-axis while the electrical beam is deflected along anX-axis. This is not necessarily so and other combinations can beapplied, including introducing mechanical movement along a first axisand deflecting the electron beam along a second axis that is not normalto the first axis. Furthermore, the direction of successive scans can bethe same or opposite of each other.

FIG. 3 illustrates a SEM 10′ that includes a pre-charge gun 100 thatprovides a tilted pre-charge beam. If both flooding beam and primaryelectron beam are applied simultaneously to substantially the same areathe working distance between the objective lens 16 and the wafer 200 maybe bigger than the corresponding working distance of SEM 10.

Smaller working distances can be achieved if both beams can be directedto locations spaced apart from each other or directed to the same areaat different time periods.

SEM 10′ is illustrated as having tilting capabilities (for tilting aprimary electron beam) but this is not necessarily so, especially if thetilt is applied to the pre-charge beam alone.

Pre-charging gun 120 is oriented such as to direct an orientedpre-charging beam towards the pre-charged object. Gun 120 can provide arelatively wide beam and can also be a flooding gun.

FIG. 4 is a flow chart of a method 300 for performing voltage contrastanalysis, according to an embodiment of the invention.

Method 300 starts by stage 310 of receiving a wafer having a first layerthat is at least partly conductive and a second layer formed over thefirst layer, following production of openings in the second layer.

Stage 310 is followed by stage 320 of directing towards the wafer afirst set of beams of charged particles (also refereed to as apre-charge beam) that are oriented at a first set of angles in relationto the wafer, whereas each of the first set of angles deviatessubstantially from normal, so as to pre-charge an area of the secondlayer without substantially pre-charging the first layer. It is furthernoted that more than a single pre-charge beam can be oriented by thesame angle.

Stage 320 is followed by stage 330 of scanning the area of the wafer byone or more second beam of charged particle beams that are oriented atone or more second angles in relation to the wafer, and collectingcharged particles scattered from the area wafer. It is further notedthat more than a single beam can be oriented by the same angle.

Conveniently, stage 330 includes introducing a mechanical movementbetween the wafer and the first beam of charged particles. Conveniently,the second angle is substantially ninety degrees.

Stage 330 is followed by stage 340 of generating an image of the area ofthe wafer in response to the collected charged particles.

According to an embodiment of the invention the intensity of the firstbeam of charged particles differs from the intensity of the second beamof charged particles.

According to an embodiment of the invention the first beam of chargedparticles is generated by a flooding gun. According to anotherembodiment of the invention the first and second beam of chargedparticles are generated by the same source, such as the electron gun ofa SEM. According to yet another embodiment of the invention acombination of both folding gun and SEM can be used.

Conveniently, stage 330 is preceded by a stage of introducing amechanical tilt between the wafer and a charged particle beam source.

Conveniently, stage 310 is preceded by stage 305 of determining thefirst angle in response to an aspect ratio of at least one contact hole.This can be contact hole having the smallest aspect ratio, but this isnot necessarily so.

Conveniently, the first layer includes electrically floating chargeableelements.

According to another embodiment of the invention the current of theprimary electron beam can be altered so that during a scan and image aprimary electron beam characterized by a first current is used to scanthe wafer while the discharge portion includes scanning the wafer with aprimary electron beam characterized by another current. Higher dischargecurrents can reduce the length of the discharge period.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment. Rather, it is intended to cover variousmodifications within the spirit and scope of the appended claims.

1. A method for electrically testing a wafer, the method comprising:receiving a wafer having a first layer that is at least partlyconductive and a second layer formed over the first layer, followingproduction of openings in the second layer; directing towards the wafera first set of beams of charged particles that are oriented at a firstset of angles in relation to the wafer, whereas the each angle of thefirst set of angles deviates substantially from normal, so as topre-charge an area of the second layer without substantiallypre-charging the first layer; scanning the area of the wafer by a secondset of beams of charged particle beam that are oriented at second set ofangles in relation to the wafer, and collecting charged particlesscattered from the area wafer.
 2. The method of claim 1 furthercomprising generating an image of the area of the wafer in response tothe collected charged particles.
 3. The method of claim 1 wherein thestage of scanning comprises introducing a mechanical movement betweenthe wafer and the first beam of charged particles.
 4. The method ofclaim 1 wherein an angle that belongs to the second set of angles issubstantially ninety degrees.
 5. The method of claim 1 wherein anintensity of a first beam of charged particles differs from an intensityof a second beam of charged particles.
 6. The method of claim 1 whereinthe stage of directing comprises generating a first beam of chargedparticles by a flooding gun.
 7. The method of claim 1 whereas the stageof scanning is preceded by a stage of introducing a mechanical tiltbetween the wafer and a charged particle beam source.
 8. The method ofclaim 1 comprising generating a first and a second charged particle beamby a certain beam source.
 9. The method of claim 1 further comprising astage of determining the first set of angles in response to an aspectratio of at least one contact hole.
 10. The method of claim 1 whereinthe first layer comprises electrically floating chargeable elements. 11.A system for electrically testing a semiconductor wafer, the systemcomprising: at least one charged particle beam source; at least onedetector adapted to collect charged particles scattered from the wafer;whereas the wafer comprises a first layer that is at least partlyconductive and a second layer formed over the first layer, followingproduction of openings in the second layer; whereas the system isadapted to: (i) direct towards the wafer at least a first set of beamsof charged particles that are oriented at a first set of angles inrelation to the wafer, whereas each angle of the first set of anglesdeviates substantially from normal, so as to pre-charge an area of thesecond layer without substantially pre-charging the first layer; (ii)scan the area of the wafer by a second set of beams of charged particlebeam that are oriented at a second set of angles in relation to thewafer, and collect charged particles scattered from the area wafer. 12.The system of claim 11 further comprising an image processor, coupled tothe at least one detector, adapted to generate an image of the area ofthe wafer in response to the collected charged particles.
 13. The systemof claim 11 wherein the system comprises a stage for introducing amechanical movement between the wafer and the first beam of chargedparticles.
 14. The system of claim 11 wherein a second angle issubstantially ninety degrees.
 15. The system of claim 11 wherein anintensity of a first beam of charged particles differs from an intensityof a second beam of charged particles.
 16. The system of claim 11wherein the at least one particle beam source comprises a flooding gunadapted to generate a first beam of charged particles.
 17. The system ofclaim 11 adapted to introduce a mechanical tilt between the wafer and acharged particle beam source.
 18. The system of claim 11 whereas the atleast one charged particle beam source comprises a single chargedparticle beam source adapted to generate the first and second chargedparticle beam by a certain beam source.
 19. The system of claim 11adapted to determine the first set of angles in response to an aspectratio of at least one contact hole.
 20. The system of claim 11 whereinthe first layer comprises electrically floating chargeable elements. 21.The system of claim 11 further comprising an electrode located at avicinity of the wafer.